The invention relates to a magneto-optical recorder comprising an optical system for directing a radiation beam towards a thermo-magnetic layer of a magneto-optical record carrier, which optical system comprises a focussing device for focussing the radiation beam so as to form a focal spot on the thermo-magnetic layer, and a servo system for keeping the focal spot in focus on said layer, which servo system is constructed to produce a correction signal which is representative of the position of the thermo-magnetic layer relative to the optical system, and a current modulator for modulating the current through a coil of a magnetic recording head.
Such a recorder is known from the article "Erasable magneto-optical recording" by M. Hartmann et al. in Philips Technical Review, Vol. 42, No. 2, August 1985, pp. 37-43.
In view of the heat dissipation in the coil and the power consumption of the recorder it is desirable that the current through the coil during a write operation is the minimum current permissible for a reliable recording on the carrier. Technical progress has led to the use of large-diameter discs with plastics substrates which are not perfectly flat and which exhibit an excursion of approximately .+-.0.5 mm parallel to their axis.
As a result of this the strength of the magnetic field at the disc surface varies from point to point depending on the axial excursion of said disc.
The currently available thermo-magnetic materials require a magnetic field of approximately 200 to 300 Oe for the purpose of recording. For a reliable recording process larger than normal currents have to be used in order to ensure that the field strength is always adequate at any point, taking into account the disc excursion. This solution also has the aforementioned drawback that the heat dissipation in the coil and the power consumption of the recorder increase, resulting in excessive field strengths at certain points of the disc, which gives rise to write noise and a consequent deterioration of the signal-to-noise ratio.
It is the object of the invention to mitigate these drawbacks.